Four observed El Niño-Southern Oscillation (ENSO) events are studied to
determine the mechanisms responsible for the anomalous extratropical
atmospheric circulation during northern winter. A parallel analysis of a GCM's
response to El Niño is performed in order to assess if similar
mechanisms are operative in the model atmosphere. The observed stationary wave
anomalies over the Pacific/North American (PNA) region are found to be similar
during the four winters despite appreciable differences in sea surface
temperatures. The anomalous transient vorticity fluxes are remarkably robust
over the North Pacific during each event, with an eastward extension of the
climatological storm track leading to strong cyclonic forcing near 40°N,
150°W. This forcing is in phase with the seasonal mean Aleutian trough
anomaly suggesting the importance of eddy-mean flow interactions. By
comparison, the intersample variability of the GCM response over the PNA region
is found to exceed the observed inter-El Niño variability. This stems
primarily from a large variability in the model's anomalous transients over the
North Pacific.

Further analysis using a linear stationary wave model reveals that the
extratropical vorticity transients are the primary mechanism maintaining the
stationary wave anomalies over the PNA region during all four observed ENSO
winters. In the case of the GCM, the organization of transient eddies is ill
defined over the North Pacific, a behavior that appears more indicative of
model error than the unpredictable component of seasonal mean storm track
anomalies. A physical model is proposed to explain the robustness of the
tropical controlling influence of the extratropical transients in nature. A
simple equatorial Pacific heat source directly forces a tropical anticyclone
whose phase relative to the climatological tropical anticyclone leads to an
eastward extension of the subtropical jet stream. This mechanism appears to be
equally effective for a beat source located either in the central or eastern
Pacific basin.